[mu4e bug] thread folding hides the next unrelated message in headers view

[danielfleischer]

Describe the bug

Newly added thread folding also hides the next message in the list which is not related to the folded thread.

How to Reproduce
Open some mu4e headers view. Fold a thread and see an additional, unrelated message being hidden.

Environment
MacOS, Emacs 30, mu4e 1.11.4.

Checklist

• you are running either an 1.8.x/1.10.x release or master (otherwise please upgrade)
• you can reproduce the problem without 3rd party extensions (including Doom/Evil, various extensions etc.)
• you have read all of the above

[progfolio]

looks like an off-by-one error when determining the boundary between the thread to fold and the next thread. This change fixed it for me, but I haven't had a chance to look over the implications outside of the fix:

(defun mu4e-thread-next()
  "Get the root of the next thread (if any)."
  (save-excursion
    (when (mu4e-thread-goto-next)
      (1- (mu4e-thread-root)))))

[djcb]

Fixed in master.